Update to v4.19 kernel headers.

Test: Builds and boots.
Change-Id: I99a9ed79666e143b47f02ca4e59eed94f69b7e4a
diff --git a/libc/kernel/uapi/linux/kernel-page-flags.h b/libc/kernel/uapi/linux/kernel-page-flags.h
index a1212cc..afa0393 100644
--- a/libc/kernel/uapi/linux/kernel-page-flags.h
+++ b/libc/kernel/uapi/linux/kernel-page-flags.h
@@ -44,4 +44,5 @@
 #define KPF_BALLOON 23
 #define KPF_ZERO_PAGE 24
 #define KPF_IDLE 25
+#define KPF_PGTABLE 26
 #endif